Contact Multiple-Effect Evaporator

It is sometimes possible to close an odorous system in order to prevent the release of the odor to the atmosphere. For example, a multiple-effect evaporator can be substituted for an open contact condenser on a process emithng odorous, noncondensable gases. 

Pulp and paper Black liquor Multiple-effect evaporators Indirect-contact digester... 

In multiple-effect evaporators the vapors from the last effect are usually leaving under vacuum, i.e., at less than atmospheric pressure. These vapors must be condensed and discharged as a liquid at atmospheric pressure. This is done by condensing the vapors using cooling water. The condenser can be a surface condenser, where the vapor-to be condensed and the cooling liquid are separated by a metal wall, or a direct contact condenser, where the vapor and cooling liquid are mixed directly. 

If an evaporator, fed with steam at 399 K with a total heat of 2714 kJ/kg, is evaporating water at 373 K, then each kilogram of water vapour produced will have a total heat content of 2675 kJ. If this heat is allowed to go to waste, by condensing it in a tubular condenser or by direct contact in a jet condenser for example, such a system makes very poor use of steam. The vapour produced is, however, suitable for passing to the calandria of a similar unit, provided the boiling temperature in the second unit is reduced so that an adequate temperature difference is maintained. This, as discussed in Section 14.2.4, can be effected by applying a vacuum to the second effect in order to reduce the boiling point of the liquor. This is the principle reached in the multiple effect systems which were introduced by Rillieux in about 1830. 

One important component of any evaporator installation is the equipment for condensing the vapour leaving the last effect of a multiple-effect unit, achieved either by direct contact with a jet of water, or in a normal tubular exchanger. If M is the mass of cooling water used per unit mass of vapour in a jet condenser, and H is the enthalpy per unit mass of vapour, then a heat balance gives ... 

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